1. Field of the Invention
The invention relates to a process for preparing vinyl acetate in a heterogeneously catalyzed, continuous gas-phase process by reaction of ethylene with acetic acid and oxygen, with utilization of the heat of reaction liberated during the process.
2. Description of the Related Art
Vinyl acetate is prepared in continuous processes with recirculation of the purified product stream. In a heterogeneously catalyzed gas-phase process, ethylene reacts with acetic acid and oxygen over fixed-bed or fluidized-bed catalysts which generally comprise palladium and alkali metal salts on a support material and can additionally be doped with gold, rhodium or cadmium.
The starting materials ethylene, oxygen and acetic acid are reacted in an exothermic reaction, generally at a pressure of from 1 to 30 bar (pressure values here and in the following in bar gauge) and a temperature of from 130° C. to 200° C. in a fixed-bed tube reactor or fluidized-bed reactor to form vinyl acetate:C2H4+CH3COOH+0.5O2=>CH3COOCH═CH2+H2O
The ethylene conversion is about 10%, the acetic acid conversion is from about 20 to 30% and the oxygen conversion is up to 90%.
In the preparation of vinyl acetate, a gas mixture consisting predominantly of ethylene, carbon dioxide, ethane, nitrogen and oxygen (recycle gas which generally contains from 60 to 70% by volume of ethylene) is circulated. The gas stream is admixed with the reactants acetic acid, ethylene and oxygen upstream of the fixed-bed tube reactor or fluidized-bed reactor and brought to the reaction temperature by means of heat exchangers operated by means of heating steam. The enrichment of the recycle gas with acetic acid is usually effected by means of an acetic acid saturator or acetic acid vaporizer heated by means of heating steam. After the reaction, the reaction products and unreacted acetic acid are condensed out from the recycle gas and passed to work-up. Product which does not condense out is scrubbed out in a scrubber operated using acetic acid.
The recycle gas or part thereof is freed of carbon dioxide formed before it is once again admixed with the starting materials.
The products vinyl acetate and water and also unreacted acetic acid which have been condensed out are separated from one another in a multistage, usually steam-heated, distillation process. The usual distillation steps are dewatering, azeotropic distillation, pure distillation, by-product removal, wastewater purification, residue work-up and low-boiler and high-boiler removal. The production plants for the work-up of the vinyl acetate can vary.
The reaction temperature in the fixed-bed tube reactor or fluidized-bed reactor of from 130° C. to 200° C. is set by means of evaporative water cooling at a pressure of from 1 to 10 bar. This forms steam, known as process-generated steam, having a temperature of from 120° C. to 185° C. and a pressure of from 1 to 10 bar, preferably from 2.5 to 5 bar. The steam can sometimes be somewhat superheated. In this case, the temperature is higher than the boiling point at the respective process-generated steam pressure. This process-generated steam can then be used for heating further process steps of the vinyl acetate preparation, for example for heating individual distillation columns for the fractionation of the product mixture. Such a procedure is described in JP-A 02-091044.
The reaction temperature is set via the operating pressure of the evaporative water cooling and the process-generated steam formed. The decrease in activity of a catalyst over the operating time is compensated by increasing the reaction temperature, i.e. the operating pressure of the evaporative water cooling and of the process-generated steam formed. The reaction temperature and thus the process-generated steam temperature thus vary over time, which leads to utilization problems with the process-generated steam. To spare the catalyst, to optimize the selectivity and to minimize carbon dioxide formation, the vinyl acetate reaction is operated for as long as possible at a low reaction temperature, corresponding to a low process-generated steam pressure.
A disadvantage here is that the process-generated steam can only be used for heating some of the pressure steps because of its low temperature and pressure level. These are, for example, the dewatering column, wastewater purification, the residue work-up which is usually operated under vacuum, a recycle gas heater and various acetic acid vaporizers and heaters. For the further process steps such as azeotropic distillation or pure distillation, external, higher-grade and often superheated heating steam has to be introduced, usually at a temperature of from 160° C. to 250° C. and a pressure of from 5 bar to 15 bar. A further disadvantage is that cooling of the process reactor in which the exothermic gas-phase reaction takes place produces more process-generated steam than can be consumed in the process steps of vinyl acetate preparation and purification because of the pressure and temperature level of the process-generated steam. It is usually possible to consume only from about 75 to 80% by weight of the process-generated steam formed for heating in process steps. The use of the process-generated steam for process steps depends strongly on the apparatus dimensions selected and the pressure level of the heating steam used for operation of the plant.
The remaining amount can either be condensed, which leads to a complete loss of the energy, or alternatively can be passed on to other operations in an integrated works. However, this is complicated in terms of organization and apparatus. In addition, low-pressure steam is mostly used for heating of selected product pipes or buildings, is therefore subject to fluctuations over the year and can therefore often not be completely utilized further.